The application of carbon materials in anode materials titanium acid lithium battery is described

Lithium titanium with structure is considered as one of the most promising cathode materials for lithium ion batteries due to its high cycle life and safety characteristics. However, the application of lithium titanium is greatly limited because of its low electronic conductivity and easy gas bilge during the charging and discharging cycle. Carbon materials with high conductivity, environmental friendliness, stable chemical and thermal properties and diverse structures are combined with lithium titanium to form a composite anode material, which can effectively improve the conductivity of the material and inhibit gas bilge, playing a key role in the optimization of the performance of electrode materials. Carbon materials in recent years was reviewed in this paper the application and research progress in lithium titanium cathode, in-depth analysis and discussion on the carbon materials on the improvement of the comprehensive electrochemical properties of lithium titanium way and improvement effect, points out the different forms of lithium titanium/carbon composite materials need to pay attention to problems in the preparation and application, and the lithium titanium/carbon composite material application direction of the future is prospected.

The introduction

With the continuous depletion of fossil energy and the aggravation of environmental problems, the development and efficient storage of clean and renewable energy have become an important issue of global concern. Among many energy storage devices, lithium ion battery has been rapidly developed and widely used since its commercialization due to its advantages such as high energy density, long cycle life, low self-discharge rate, no memory effect and environmental friendliness. In recent years, in order to adapt to the rapid development of new energy vehicles, smart grid and other large energy storage devices, the development of lithium ion batteries with high energy density, high power density, excellent safety and long cycle life has become a focus of attention in the field of energy storage.

At present, the commercialized cathode materials of lithium ion battery are still mainly carbon materials (such as graphite) that can be reversibly embedded and exfoliated lithium ions between layers. Because the lithium potential of graphite is close to that of lithium metal, lithium dendrite will be formed during overcharge, which causes safety problems. In the process of lithium inlay, graphite materials will also lose effective conductive contact between active materials due to volume expansion, resulting in capacity loss. These shortcomings limit the application of graphite carbon anode materials in vehicle and smart grid and other large lithium ion batteries and power batteries. Therefore, it is the key to develop the next generation of lithium ion batteries to find anode materials that are safer and more reliable and have longer cycle life than carbon anode materials.

Lithium titanium with structure is one of the new cathode materials. Compared with the graphite anode, lithium titanium has a higher lithium potential, which can effectively avoid the precipitation of lithium metal and the formation of lithium dendrites. Lithium titanium and lithium embedded Li7Ti5O12 have much higher thermodynamic stability than graphite, which is not easy to cause thermal runaway of the battery, thus having higher safety. At the same time, in the process of lithium ion implantation and release, the crystal lattice parameters hardly change, and the crystal structure can maintain a high degree of stability. As a "zero strain" material, lithium titanium has excellent cyclic stability. In addition, lithium titanium also has excellent low-temperature performance, rapid charging capacity and high cost performance, so it has a good application prospect in large-scale energy storage and other fields.

Although lithium titanium has the above advantages, since lithium titanium was first used as a negative electrode material in 1989, lithium ion batteries with lithium titanium as a negative electrode have not achieved the expected rapid industrialization. The main factors restricting the large-scale application of lithium titanium are from the two aspects of materials and devices:

First, the electronic and ionic conductivity of lithium titanium are respectively 10-13s ˙ And the 10-9-10-13 cm - 1 cm2 & dot; S-1, the energy gap of electron transition in lithium titanium is about 2eV, and the intrinsic insulation property of the material greatly limits its multiplier performance under the condition of large current charge and discharge.

Second, in the charging and discharging cycle and storage process of lithium ion batteries with lithium titanium as the negative electrode, there is a general phenomenon of "flatulence", that is, gas is constantly generated inside the battery, especially at high temperature, flatulence is more serious.

In view of the above two problems, domestic and foreign researchers have carried out extensive and in-depth research work, including:

(1) improve the electronic conductivity of lithium titanium by ion doping, coating of conductive substances or forming composite materials with conductive substances;

(2) NANO structure design of lithium titanium is carried out to shorten the ion diffusion distance and improve the multiplier performance;

(3) through in-depth study of the gas generation mechanism of lithium titanium materials, to find ways to inhibit battery gas bloating. In recent years, lithium titanium has become a hot topic in the research of combining carbon materials with lithium titanium, which have high conductivity, environmental friendliness, stable chemical and thermal properties and diverse structures, with lithium titanium to form composite materials.

In recent years was reviewed in this paper the application of carbon material anode materials in lithium titanium research, in-depth analysis and discussion on the carbon material effect on the improvement of the comprehensive electrochemical properties of lithium titanium, lithium titanium/carbon composites are summarized in the problems need attention in the preparation and application, and finally points out the lithium titanium/carbon composite electrode materials application direction of the future

Basic structure and charge and discharge mechanism of lithium titanium

Lithium titanium as the white crystal with structure, can be stable in the air, the formula can be written as Li (Li/ 3 ti5/3), m1 for Fd3m space lattice group, the oxygen ions cubic thick pile of a face-centered cubic lattice, located at 32 e position, lithium ion tetrahedron 8 a position, and 16 d position respectively by lithium ion and titanium ion (Ti4 +) with 1, 5 atomic scale. Therefore, the structural formula of lithium titanium can be expressed as Li(8a)[Li/ 3ti5/3](16d)O4(32e). In lithium ion embedded process, three original 8 a position of lithium due to electrostatic repulsion is transferred to 16 c position, at the same time, three new lithium atoms are embedded into the structure of 16 c position, which happen to rock salt structure Li7Ti5O12 Li4Ti5O12  structure transformation, that is accompanied by three Ti4 + to Ti3 +, including Li7Ti5O12 formula can be represented as Li2 (16 c) Li / 3 ti5/3 (16 d) m1 (32 e).

It is generally believed that the charge and discharge mechanism of the two-phase transition follows the core-shell model shown in figure 2, that is, when lithium is embedded, the surface of lithium titanium particles gradually forms a highly conductive Li7Ti5O12 layer of the salt phase. When lithium is embedded, Li4Ti5O12 is completely transformed into Li7Ti5O12. When lithium comes out, the surface of Li7Ti5O12 is gradually covered by the formed low electronic conductance Li4Ti5O12, and finally Li7Ti5O12 is completely transformed into Li4Ti5O12. A very flat potential platform with a potential of 1.55V(Li/Li+) exists during the lithium imbed/stripping process. During this process, the cell parameters changed from 8.3595 to 8.3538, and the corresponding cell volume change was only 0.2%, which could be almost ignored. Therefore, lithium titanium is also called "zero-strain" material. This characteristic of lithium titanium makes it have very high structure stability and provides the foundation for its good charging-discharging cycle performance.

Improvement of the performance of lithium titanium by carbon materials

Although the structural characteristics of lithium titanium enable it to have better cyclic stability and safety than graphite carbon materials, the electronic conductivity of lithium titanium is very low, which makes it difficult to fully exert the specific capacity of the material. At the same time, the performance of lithium titanium under the condition of large current charge and discharge is limited, which can not meet the requirements of high-power batteries. In order to improve the ratio of lithium titanium performance, in the recent 20 years, researchers have designed and developed a variety of methods, such as through the study of the NANO structure of lithium titanium material designed to shortening the distance between ion diffusion in the bulk phase, or through the surface modification and ion doping to improve lithium titanium surface and phase conductivity, or combine several methods to improve the comprehensive performance of materials. Carbon materials with high conductivity, environmental friendliness and diverse structures can be combined with lithium titanium materials through coating, uniform mixing, in-situ composite and other methods to effectively improve the conductive contact between lithium titanium particles and enhance their conductivity. At the same time, through design and construction, multiple forms and structures of carbon materials and lithium titanium are used to form a composite structure conducive to ion transport, which can also improve the migration rate of lithium ions and electrolyte in the electrode materials, thus improving the rate performance and cycling stability of the electrode materials.

Carbon coated lithium titanium

Carbon coating is currently the most commonly used lithium titanium modification methods, can be directly introduced in the process of preparation of carbon source, the lithium titanium particles surface thickness of carbon coated layer, so as to enhance the electrical conductivity of lithium titanium material surface, and improve lithium titanium and collection of fluid between the conductive contact between particles, improve the electrochemical performance of lithium titanium. Carbon coating of lithium titanium can be achieved by solid phase method, hydro-thermal method, sol-gel method and other methods. The carbon sources used are various, including inorganic carbon sources, organic carbon sources, and carbon deposited by chemical vapor phase method. In recent years, with the deepening of research, nitrogen-doped carbon is also used as a coating layer to further improve the electrical conductivity of materials.

Wang at all. took PANI as carbon source and prepared lithium titanium NANO particles with conductive Ti3+ and carbon layers on the surface by in-situ coating method. In-situ carbide of PANI can not only inhibit the growth of lithium titanium particles and play a role in particle size regulation, but also reduce part of the material surface Ti4+ to Ti3+, which, together with the formed carbon layer, increases the electrical conductivity of the material, thus significantly improving the rate performance. When the current density increases to 1.5a /g(about 10C), the discharge capacity can reach 115mAh/g.

Jung at all. used asphalt as carbon source and prepared carbonaceous lithium titanium porous micron-sized spheres by simple solid phase method. Each micron ball is composed of several NANO meter lithium titanium particles, so the material has a high Tap density. At the same time, the NANO pore structure formed between the NANO particles is conducive to the rapid transfer of electrolyte and lithium ions, while the uniform carbon coating improves the conductivity of the material, so that the material shows excellent magnification performance. At the magnification of 100C, the discharge specific capacity of 123mAh/g is still 70.3% of the theoretical specific capacity.

Li at all. used the CTAB as surfactant and carbon source, adopted hydro-thermal and subsequent heat treatment methods, and obtained carbon-coated lithium titanium with high magnification performance. Under 10C and 20C charge-discharge magnification, the specific capacity could reach 151mAh/g and 136mAh/g, respectively.

Cheng at all. used the method of thermal phase decomposition to carry out an effective carbon coating on lithium titanium particles, and formed a continuous and uniform (about 5nm) carbon layer on the surface of lithium titanium. Raise cm-1 to 2.05S˙ Cm - 1. At the current density of 0.2ma /cm2, the discharge specific capacity was 155mAh/g. The author also compared the electrochemical reaction pathways between the electrode materials formed by mixing carbon black as a conductive agent with lithium titanium and the carbon-coated lithium titanium, as shown in figure 3. Although most areas on the surface of particles of the former can contact with the electrolyte, which is conducive to the diffusion of lithium ions, the actual reaction is limited to the sites with electron conduction paths. The carbon cladding layer of the latter can provide enough electron conduction paths for the material, and the crystal defects in the carbon cladding layer can facilitate the passage of lithium ions through the carbon layer, so the carbon-coated lithium titanium shows higher multiplier characteristics.

For the first time, Zhao at all. prepared nitrogen-doped carbon-coated NANO lithium titanium particles by using Ionic liquid methyl as the carbon source. Since the addition of nitrogen can increase the reactivity and conductivity of carbon, the material has a good rate characteristic, and can obtain the discharge specific capacity of 130mAh/g at 10C rate.

Li at all. in our research group took TBAOH as dispersing agent, carbon source and nitrogen source, and after heat treatment, carbon coating layer of nitrogen doped was formed uniformly on the surface of ultra-thin two-dimensional lithium titanium NANO sheet about 1-2nm thick. The nitrogen-doped carbon layer uniformly coated on the surface of the lithium titanium NANO sheet can form an effective conductive network in the whole electrode, which is conducive to the rapid transmission of electrons, while the abundant pore structure is conducive to the diffusion of lithium ions and electrolytes, so that the electrode shows a super high magnification performance. At 100C, the reversible specific capacity is still as high as 131mAh/g.

In addition to the carbon source, the method steps used for carbon cladding are also critical. The preparation of lithium titanium with high crystallization usually requires a high temperature heat treatment process, but the high temperature heat treatment often leads to the destruction of the NANO structure of lithium titanium precursor due to the hardening agglomeration, which cannot be retained. Aiming at this characteristic, Li at all. proposed a method to prepare carbon coated NANO titanium oxide in advance, and then react it with lithium salt at high temperature to generate carbon coated lithium titanium. Lithium titanium materials with both NANO structure and high crystallization can be obtained by this method. In addition to improving the electrical conductivity of the materials, the preformed carbon coating can also inhibit the growth of grains in the subsequent high-temperature reaction and play a role in limiting particle size. By using this method, Zhu at all. firstly prepared the carbon-coated titanium oxide by sucrose dissolution, and then obtained the carbon-coated NANO porous micro spheres by grinding, spray granulation and calcining with lithium salt pellets. The material still has a specific capacity of 126mAh/g at the rate of 20C, and 1000 cycles at the rate of 1C, with a capacity retention rate of 95%. Shen at all. also used this method to preform NANO titanium oxide with carbon, and then make it react with lithium salt in high temperature solid phase reaction, and finally form carbon coated lithium titanium with core-shell structure, in which the particle size of lithium titanium is about 20 ~ 50nm, and the thickness of carbon layer is about 1 ~ 2nm. The lithium titanium of this structure showed excellent magnification performance, with a specific capacity of 85.3mAh/g at the magnification of 90C and a capacity retention rate of 95% after 1000 cycles at the magnification of 10C.

Table 1 summarizes the effects of different coating methods, carbon content, carbon layer thickness and graphite degree on the properties of carbon-coated lithium titanium. The degree of graphite of carbon materials in the table is represented by the peak strength ratio (ID/IG) of D film and G film in the Raman spectrum. The smaller the ratio is, the higher the degree of graphite is. As can be seen from the table, when carbon coating improves the electrochemical properties of lithium titanium materials, its effect will be affected by factors such as the degree of graphite, thickness and uniformity of carbon coating.

Due to the formation of carbon coated carbon source diversity, different structure, make the resulting carbon coated layer graphite degree of carbon materials is not the same, and the graphite degree of carbon materials determines its high and low electrical conductivity, so the coating effect tend to have very big difference, need to be done according to the requirements of material properties and the choice of carbon source and optimization; Carbon coating layer thickness and material of the closely related electronic conductivity and ionic diffusion rate, which increase the carbon coating layer thickness on the surface of the material, material surface and so will the conductivity of the corresponding between particles increase, but a thick layer of carbon coated and inhibition of the lithium ion transport, to a certain extent reduce the ion diffusion efficiency, thus influence on the performance ratio; The uniformity of carbon cladding directly affects the chemical and physical properties of the material surface, including electron transfer, ion diffusion and the formation and stability of SEI film on the material surface. Zhu, such as coating layer thickness was studied and the graphite degree of carbon coated titanium acid lithium materials the influence of the electrochemical properties, the study found that the charge transfer impedance Rct and lithium ion diffusion coefficient would be increased with the increase of the carbon coating layer thickness is reduced, but the carbon coating layer thickness on the lithium ion transport rate is bigger, the influence of so carbon coating layer should be as thin as possible. In addition, the improvement of graphite degree can increase the conductivity of the material and reduce the defects at the same time. However, due to the reduction of its own structural defects, the transmission rate of lithium ion will also be reduced accordingly. Chen at all. reached similar conclusions, believing that the graphite degree of carbon cladding layer and the increase of carbon layer thickness would hinder the diffusion of lithium ions to some extent. In conclusion, when carrying out carbon cladding on the surface of lithium titanium, not only a reasonable selection of carbon source should be made, but also the thickness and uniformity of carbon cladding layer should be regulated to improve the electron transfer and ion diffusion rate of the material at the same time, so as to obtain the best electrochemical properties of the material.

The performance improvement of lithium titanium by carbon coating was further confirmed in the all-battery test. Zhu at all. assembled the carbon-coated NANO pore lithium titanium and lithium (LiMn2O4) into 26650 columnar lithium battery by using the prepared carbon-coated NANO pore lithium titanium and lithium (LiMn2O4). The capacity of 2600mAh (specific energy density 70Wh/kg) at 0.2C(520mAh/g) can be utilized. They believe that the main cause of capacity decay is that the dissolution of manganese in the electrolyte destroys the integrity of the SEI film on the carbon surface. To test this idea, they assembled carbon-coated NANO pore lithium titanium and lithium iron phosphate into a 18650 columnar battery that cycles 3,000 times and has almost no attenuation in capacity. He at all. assembled lithium titanium coated with carbon and lithium titanium coated with nickel cobalt manganese (NCM) ternary anode materials into a type 034352 soft-pack battery, respectively, and carried out a comparative study. They found that after 400 cycles at 0.5c multiplier, the capacity of the carbon-coated lithium titanium battery had no attenuation, while the capacity of the carbon-coated lithium titanium battery had a attenuation of 6.9%. Wen at all. assembled the carbon-coated lithium titanium micro sphere prepared by them and the lithium positive electrode into a type 043048 square-shell battery. Through the study, it was found that the carbon-coated lithium titanium micro sphere had a stable interface and good electrical conductivity, and the rate and cycle performance of the assembled battery were significantly better than those of the carbon-coated lithium titanium micro sphere. The improvement of the performance of the carbon-coated lithium titanium battery is also attributed to the improvement of the material interface and battery gas expansion by the carbon-coated lithium titanium as the negative electrode, which will be described in detail in section 4. Based on the above research results, carbon-coated lithium titanium materials have been partially commercially produced and applied.

Lithium titanium/carbon composite with special structure Carbon materials can be used as coating to improve the surface electrochemical properties of lithium titanium, and can also be combined with lithium titanium to form composite materials with special morphological structures according to their own advantages in structure and performance. In recent years, with the deepening of the research on carbon NANO tubes, graphite and other new NANO carbon materials, these carbon materials with excellent conductivity, high specific surface area, light weight and flexibility are increasingly used in the preparation of composite electrode materials, and show great application prospects.

Carbon NANO tubes have unique one-dimensional tubular NANO structures, ultra-high aspect ratio, excellent electrical conductivity, large specific surface area, high mechanical strength and good chemical stability. After the formation of composite material with lithium titanium, the conductive contact between the active material particles and the active material and the collector fluid can be increased, thus the electrical conductivity of the material can be significantly improved. Shen at all. prepared a composite material of more wall carbon NANO tubes and lithium titanium with coaxial NANO tether core-shell structure by means of sol-gel method, subsequent water heat treatment, calcining and other processes, and its schematic diagram is shown in figure 5. The "core" of the material is a more wall carbon NANO tube with high conductivity, which can provide a good conductive connection, while the "shell" is composed of lithium titanium NANO particles, forming a porous structure about 25nm thick, which can provide a large electrode/electrolyte contact interface for the material, thus shortening the diffusion distance of lithium ions. Compared with lithium titanium NANO particles, lithium titanium composites with NANO tether core-shell structure have higher electrical conductivity and lithium ion implantation kinetics, thus obtaining higher rate performance and cycling stability. The specific capacity of 96.1mah /g and 68mAh/g remained at 40C and 60C magnification respectively, and only 5.6% of the capacity was attenuated after 100 cycles at 1C magnification.

Naoi at all. synthesized a kind of composite material with lithium titanium NANO particles growing uniformly on the outer wall of carbon NANO fibers by using carbon NANO fibers. It has a super high magnification performance.

Graphite is made of the single layer of carbon atoms with two dimensional structure of NANO carbon materials, besides having the excellent properties, carbon NANO tubes have its special two-dimensional structure and higher strength, make it has good flexibility, can be in the composite materials have the effect of structural support and buffer volume expansion, so the addition of graphite can make poor conductivity and volume expansion type comprehensive electrochemical properties of electrode materials have improved significantly, is widely used in a variety of composite electrode materials. Shen at all. prepared a composite electrode material of NANO meter lithium titanium crystal particles anchored to graphite NANO meter sheets by wet chemical method. In graphite and the process of in situ composite lithium titanium, graphite can be formed between the electrode materials not only high conductivity network structure, but also in the process of material preparation, inhibit particles grew up with together, can not only improve the whole conductivity of the material, also shortened the lithium ions and electrons in the material transport path, so that the material has good performance ratio, under 60 c ratio, the specific capacity is 82.7mAh/g.

Zhu at all. prepared graphite-coated lithium titanium NANO particle composites by combining sol-gel method and electrostatic spinning technology. Electrostatic spinning technology enables the composite to form a NANO scale structure, which is conducive to the transmission of ions and electrons, while the addition of graphite improves the conductivity of the material, so that the material has a high rate of magnification and cycling stability. At the magnification of 22C, after cycling for 1300 times, the capacity retention rate of 91% is still maintained.

Pang at all. used the method of electrostatic adsorption and assembly to prepare a composite structure material with in the hole lithium titanium particles loaded on graphite sheets. The specific capacity of the material was 141mAh/g at 10C ratio, and the capacity loss was 9% after 100 cycles.

Oh at all. prepared graphite-encapsulated lithium titanium NANO particles as a composite structure material by strengthening the phase reaction with electrostatic interaction. This material has the best magnification performance when the graphite content is 2.1%, and the specific capacity can reach 105mAh/g at 100C magnification.

Using microwave-assisted solvent thermal method, Kim at all. prepared lithium titanium NANO particles evenly distributed on the reduced go as composite electrode material, as shown in figure 6. The specific capacity of the composite material was 128mAh/g and 101mAh/g at 50C and 100C magnification respectively, and the capacity retention rate was 96% after 100 cycles at 10C magnification. Due to the addition and structural regulation of graphite, these composite electrode materials all showed significantly improved magnification performance.

Our research group has prepared NANO scale lithium titanium particles by ball milling process commonly used in industry, and dispersed them uniformly on highly conductive graphite. Fold of graphite layers overlap each other constitute the three dimensional conductive network, thus effectively reduced the charge transfer between the electrode materials and electrolyte interface resistance, and makes the electrode materials of electric potential difference significantly reduced charge and discharge platform, polarization of electrode has been effectively improved, thus obtained the excellent ratio of performance and stable cycle performance. At the rate of 30C, the specific capacity can reach 122mAh/g, and after 300 cycles at the rate of 20C, the capacity retention rate is 94.8%.

By using the high conductivity of NANO carbon materials and its own structural characteristics, a good conductive channel or network can be provided for lithium titanium in composite materials, which can overcome the deficiency of lithium titanium conductivity. However, to improve the power density of electrode materials, electron conductivity and ion diffusion rate need to match each other. Therefore, the key to the preparation of lithium titanium composite electrode materials is how to design the structure of the composite materials and improve the material conductivity and ion transport at the same time. From the perspective of application, carbon NANO tubes have been used as a conductive agent for a variety of electrode materials due to their good conductivity, and show a certain market competitiveness, more suitable for the use of lithium titanium as a conductive agent. Graphite is more likely to be used in lithium titanium composite electrode. The excellent electrical conductivity and excellent flexibility of graphite are used to build a conductive network. In addition, particle size is regulated during in-situ growth to improve the diffusion rate of ions, so as to improve the multiplier performance of composite electrode material.

Lithium titanium/carbon flexible integrated electrode

With the increasing demand of flexible and wearable electronic devices in the portable electronic products market, flexible lithium ion batteries with light weight, thin body, stretchable, deformed and bending resistance are required to be developed. As a matching power source, relevant research has become one of the hot spots in the field of electrochemical energy storage. Flexible electrode with both quick charging capacity and long cycle life is the core of flexible lithium ion battery construction, so the design and development of flexible electrode materials are of great significance. Because most carbon material with porous or network structure, large specific surface area, good electrical conductivity, high mechanical strength and good chemical stability, therefore, so far, have been reported more flexible electrode materials for lithium-ion secondary battery for film or paper of carbon materials, such as carbon NANO tube film, graphite paper, carbon/carbon fabric, etc. By combining the carbon based materials with good flexibility and high conductivity with lithium titanium to form a flexible integrated electrode, the advantages of both can be taken into account, providing a new idea for the development of flexible electrode.

Wu at all. used the oriented carbon NANO tube thin film drawn from the oriented carbon NANO tube array as the flexible framework, sprayed the suspension containing lithium titanium particles on it, covered another layer of carbon NANO tube thin film, and continued to spray. After repeated 5 times, the Li4Ti5O12/CNT flexible electrode was obtained, as shown in figure 7. In this electrode, the carbon NANO tube network provides an efficient transmission path for the electron, and its good mechanical properties ensure the mechanical strength of the electrode, so the electrode is superior to the slurry coated electrode in both electrochemical stability and mechanical strength. The flexible electrodes of lithium iron phosphate and lithium titanium prepared by this method are used as positive and negative electrodes respectively, which can be assembled into a whole battery with stable voltage output, good circulation, multiplier performance and bendable performance.

Shen at all. prepared flexible composite electrodes of lithium titanium and lithium  with carbon fabric by growing metal oxides in situ on carbon fabric and combining with chemically embedded lithium. The prepared flexible electrode showed excellent magnification performance and good cycling stability. Among them, the lithium titanium/carbon fabric composite negative electrode still had a specific capacity of 103mAh/g at 90C, and was circulated 200 times at the magnification of 10C, with a capacity loss of only 5.3%.

Carbon materials can be used not only as supporting framework of lithium titanium flexible electrode, but also as collecting fluid of lithium titanium electrode. Using carbon materials with higher conductivity and lower density, instead of metal materials as the collector fluid, can significantly improve the overall mass energy density of the electrode. Hu at all. used highly conductive carbon NANO tube membrane as the collector fluid of the electrode, took lithium titanium and lithium cobalt oxide as negative and positive materials, respectively, coated on the carbon NANO tube membrane, paper as the membrane, and PDMS as the packaging material, and assembled a flexible battery with a thickness of about 300 m. The flexible battery can be used to light the LED lamp under the condition of bending. After 50 times of bending with the radius of curvature less than 6mm, the structure is still intact, showing good mechanical flexibility.

In this research group, lithium iron phosphate and lithium titanium are grown in situ on the graphite foam surface with three-dimensional connected network structure by hydro thermal method. Flexible positive and negative electrodes that can be charged and discharged rapidly are designed and prepared, and flexible full battery is assembled, as shown in figure 8. The use of lightweight graphite foam instead of metal as the collector fluid can effectively reduce the proportion of non-active substances in the electrode, while the high conductivity and porous structure of the three-dimensional graphite network provides a rapid diffusion channel for lithium ions and electrons to achieve rapid charging and discharging performance of the electrode material. The structure and charge-discharge characteristics of the lithium ion battery assembled by the flexible positive and negative electrodes remain unchanged under the condition of repeated bending. In order to realize the practical application of flexible electrode, our research group tried to use the highly conductive and large sheet graphite exfoliated by chemical expansion as the collector fluid to prepare the integrated flexible electrode of lithium titanium/graphite and lithium iron phosphate/graphite. Through vacuum filtration, the large-sized graphite NANO sheets were overlapped with each other to form collector fluid, and the active electrode slurry was partially infiltrated into graphite to form close contact with graphite as collector fluid, effectively reducing the interface resistance of the electrode. The flexible battery can work normally in flat and bent state and has high energy density.

In the study of lithium titanium/carbon integrated flexible electrode, the researchers make use of the mechanical characteristics of carbon material itself to form a composite flexible electrode with lithium titanium through structural design. Since the loaded lithium titanium on the prepared flexible electrode itself is still a particle and does not have flexibility, when the load capacity of lithium titanium is increased, the electrode flexibility will be degraded and the electrochemical performance will decline under the bending state. Cao at all. used the membrane of carbon NANO tube/carbon fiber as the collecting fluid and lithium titanium as the electrode. When the proportion of lithium titanium in the electrode was 50%, the electrode showed good electrochemical performance, but with the increase of lithium titanium load, the cycling performance and specific capacity of the electrode decreased obviously. Therefore, only when the electrode active material itself is flexible can the limitation of carrying capacity of active material be overcome. For example, ultra-thin two-dimensional NANO sheet lithium titanium material is used to replace granular lithium titanium to prepare flexible integrated electrode. Since lithium titanium itself is a 2 d layer structure with flexibility, flexible electrode film can be formed through mutual lap, so as to overcome the limitation of load.

10. PNG

To sum up, carbon materials can improve the comprehensive electrochemical properties of lithium titanium by surface coating, forming a composite material with a specific structure, or making flexible integrated electrode, in which carbon materials mainly play the role of conductive enhancement, interface protection, particle size limitation and flexible support, as shown in table 2.

Improvement of gas expansion of lithium titanium battery by carbon materials

After modification, the improved lithium titanium material showed good electrochemical performance and application prospect, but lithium titanium as anode of lithium ion battery, in the process of charging and discharging cycle and store still prevalent problems "gas", especially under the condition of high temperature, experience constantly produce gas inside the cell, cause the battery shell deformation, performance fell sharply, severely restricted the cathode for lithium titanium battery commercialization process. Up to now, there are few reports on the gas behavior of lithium titanium electrode, and there is still no accepted conclusion on the gas generation mechanism. It is believed that the gas production of lithium ion battery with lithium titanium as the negative electrode is caused by the adsorption of water on its surface, and the water content directly affects the expansion rate. It is also believed that the gas production of lithium titanium battery is due to the reduction reaction of electrolyte on the surface of lithium titanium, which produces gases such as H2, CO2 and CO. According to the above characteristics of lithium titanium, the swelling of lithium titanium battery with negative electrode can be alleviated by means of dividing water, electrolyte optimization and surface treatment. It is a simple and efficient method to cover the active surface of electrode with surface modification and thus inhibit the gas production of lithium titanium.

Through such as He would lithium titanium pole piece respectively in the pure solvent, soaking in the electrolyte, detection of Li4Ti5O12 / Li (Ni1/3 co1/3 mn1/3) O2 (sliding) after full battery during storage and circulation of gas composition, size, bilge gas production mainly comes from lithium titanium inherent interface reaction between surface and the electrolyte solution, the solution in the outermost layers of lithium titanium crystal to take off the TS and TC and TQ reaction happened, then produce the H2, CO and CO2 gas. CO2 is not caused by the reaction of the decomposition product PF5 of LiPF6 with the electrolyte, and H2 is not produced by the reaction of lithium ions or lithium metal with trace amounts of water in the system. Through further analysis, found that when the solution and lithium titanium interface reaction occurs, will gradually form a very thin layer on the surface of the SEI film, but as a result of this kind of interface reaction rate slower, reaction will continue in the process of cell cycle and long-term storage occurs, resulting in continuous gas, it totally different from the mechanism of graphite anode formation of SEI. Graphite anode due to the formation of the SEI film on the surface of the electrolyte of reduction reaction will happen in about 0.7 V, the SEI film formed in the previous cycle, only complete stable SEI film formed by graphite and electrolyte will be completely isolated, to avoid the further reduction of electrolyte decomposition, so the battery and gas is confined to a few times before cycle, can be adjusted by optimizing the turned to technology to carry out effective control.

He at all. further found that the construction of barrier layer was an effective method to control the interface reaction between lithium titanium and the surrounding electrolyte, and the use of carbon cladding layer at the NANO meter scale could effectively inhibit the expansion reaction of lithium titanium battery. The electrolyte showed different reactivity to lithium titanium coated with carbon and lithium titanium coated. For the lithium titanium not coated with carbon, when the voltage range is 0 ~ 2.5v, the electrolyte will reduce and decompose around 0.7v, while for the lithium titanium coated with carbon, the similar reaction only occurs in the first cycle. The carbon cladding layer can cover the active sites on the surface of lithium titanium particles and form a solid electrolyte interface film to isolate the active sites from the surrounding electrolyte and avoid further reduction and decomposition of the electrolyte, as shown in FIG. 9.

In this research group, asphalt was used as PT carbon source to form a uniform coating layer on the surface of lithium titanium NANO particles with high crystallization, which was made into micro-spherical secondary particles, which were assembled into a type 043048 square shell battery with lithium positive electrode. The results show that the presence of carbon cladding layer can form a dense solid electrolyte interface film on the surface of lithium titanium and prevent the reaction between lithium titanium and electrolyte in the cycle, thus effectively inhibiting the formation of gas and the deposition of Mn. Due to the stable interface and good electrical conductivity of the carbon-coated lithium titanium micro spheres, their magnification performance and cycling stability were significantly improved. After 1000 cycles at 1C magnification, the capacity retention rate of the assembled battery is still as high as 93%, without gas expansion. However, the lithium titanium battery without carbon coating only cycles 580 times, and its capacity attenuates to 23% of its initial capacity, with obvious gas expansion, as shown in FIG. 10.

To sum up, the use of carbon coating processing, can be in the conductivity of the lithium titanium materials at the same time, improve lithium titanium and interface interaction between electrolyte, inhibition of lithium titanium as anode of lithium ion battery bilge gas, make the preparation of anode materials can meet the requirements of actual commercialization production, for the future lithium titanium industrialization application in lithium ion battery cathode brings hope.

Conclusion

Lithium titanium is recognized as the most application prospect of one of the lithium ion battery anode materials, but due to the intrinsic electronic conductivity is low, making it difficult to play high ratio of performance, in addition, easy happening between lithium titanium and electrolyte interface side effects, make the cathode for lithium titanium battery easily bilge gas problem, the commercial application of lithium titanium impeded. The use of lightweight, high electronic conductance, environmentally friendly and diverse carbon materials and lithium titanium to form various forms of composite structure electrode materials can effectively improve the electrochemical properties of lithium titanium and alleviate or eliminate the problem of gas expansion in the use of lithium titanium. Carbon plays the role of conductive enhancement, interface protection and flexible support in the composite structure electrode material formed with lithium titanium. Through reasonable design, therefore, the carbon materials with lithium titanium formed with a specific compound structure of electrode materials, such as lithium titanium surface with NANO structure uniformity of carbon coated in advance, and with high conductivity and graphite, carbon NANO tubes have a certain mechanical strength and three-dimensional structure of the composite electrode materials and carbon materials can be ensure fast ion transport at the same time, provide effective interface protection for lithium titanium, inhibit the bilge gas, and to enhance the overall electrical conductivity of the material, can also provide corresponding flexible support for lithium titanium. This composite method will be an important means to realize the commercial application of lithium titanium in the future.

With the development of basic research on lithium titanium anode, lithium ion batteries with lithium titanium as anode have been put into practical use. Lithium titanium, modified by carbon material, will be further applied in the field of hybrid electric vehicles, flexible electronic devices and large energy storage in the future due to its superior multiplier characteristics and surface chemical properties, as well as its inherent low temperature performance, ultra-high cycle life and safety. Due to the volume of a lithium titanium energy density was lower than those of graphite, also cost of graphite on the high side, and its application in the field of automotive power battery has been bigger controversy, so far, lithium titanium battery lithium battery in the global market accounts for more than 2%, but the energy technology co., LTD has been attempted to raise the energy density of lithium titanium battery and reduce the cost, together with its wide working temperature range (- 50 ~ 60 ℃), the future or in the field of public transport and military industry and other special gain greater market applications. In addition, due to its excellent safety and long cycle life, lithium titanium materials are more likely to play an important role in large energy storage power stations in the future.

The page contains the contents of the machine translation.